1. Field of Invention
This invention is concerned with encryption with an encryption/decryption system and more particularly with such a system incorporating time of day (TOD) signals to encrypt the encrypted signal.
2. Description of the Prior Art
It is common practice in data encryption and spread spectrum communications sytems to use digitally generated codes to provide data encryption and frequency spreading. The generated codes must be known at both the transmitting end (encryption station) and receiving end (decryption station) but not by the "enemy." Both linear and non-linear codes are used, with non-linear codes being used for higher security applications. Both codes are nearly impossible to break by an unsophisticated observer. However, linear codes are much easier to decipher by someone versed in the art. In many cases, codes are deciphered for code breaking purposes on a recorded basis, that is, the encrypted signal is recorded on some medium such as magnetic tape and then played back into a system containing deciphering algorithms.
Time of day encryption/decryption systems are also known which use a clock of some sort to modify the transmitted code and received code as a function of time of day (TOD). This is in addition to a modification by digital codes mentioned previously. It is the TOD encryption/decryption system upon which the invention improves.